Front-end amplifiers for the beam phase loops in the CERN PS

Alessandro Meoli (CERN BE/RF/FB)

Supervised by Heiko Damerau

21 April 2015 - CERN

Beam current structure and wall image current

DC beam current:

𝐼𝐵𝑒𝑎𝑚=𝑞𝑁𝑡

=𝑞𝑁𝑙

𝛽𝑐

Beam image current:

At relativistic velocities, the wall current has the same time structure as the beam current, so it is a mirror (except the DC component) of the beam current

How can we measure it?

𝐼𝐵𝑒𝑎𝑚

𝐼𝑤𝑎𝑙𝑙

𝐼𝐵𝑒𝑎𝑚≅− 𝐼𝑤𝑎𝑙𝑙

[1] Denard, J.C. “CERN Accelerator School on Beam Diagnostics”, 28 May -6 June 2008, Dourdan

[1]

[1]

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Wall current monitor

The H field outside the beam pipe is 0, according to the Ampere’s law:

We can measure the wall current, cutting the beam pipe, and inserting a ceramic break, to force the current through an impedance.

[2]

[2] D’Elia, A “Status of the Wall Current Monitor Design for EUROTeV”, CERN2

Actual design

Very high dynamic range (mV to hundreds V)

Mechanical relay, switches every low intensity beam cycle!

We need different levels of attenuation

• Bad reliability over time• Difficult to chose the correct intensity range

WCM

PS SS95

LHCPROBE Beam 5*

TOF Beam 7.5*

More than 3 orders of

magnitude!

Injection

Extraction

3

𝑝𝑎𝑟𝑡𝑖𝑐𝑙𝑒h𝑏𝑢𝑛𝑐

𝑝𝑎𝑟𝑡𝑖𝑐𝑙𝑒h𝑏𝑢𝑛𝑐

New design

0 dB

5 dB

10 dB

20 dB

30 dB

WCM

40 dB

50 dB

60 dB

50 Ω Multiplexer

The idea is to design a circuit similar to the front end of an oscilloscope:• Large bandwidth• High input impedance• Protection against overvoltage

High dynamic rangemV up to 800V

Small dynamic range (+- 5V)

Design specification:• Bandwidth: 100KHz-100MHz (Highest

frequency of interest for the phase loop is 80MHz)

• High input impedance, to perturb the signal as less as possible

• High sensitivity but at same time protection against overvoltage

Signal distributor

Future option

4

New design

High input impedance attenuator

Input stage Bandwidth limiter

Output buffers

50 Ω

50 Ω

50 Ω

50 Ω

5

High impedance input attenuator stage

• Finite input capacitance of the amplification stage• Resistive divider becomes a low-pass filter• Need to compensate:

𝑅𝑠𝐶𝑠=𝑅𝑖𝐶𝑖

Fine tuning in lab, to compensate the parasitic capacitance

6

• Low frequency: resistive divider• High frequency: capacitive divider

Prototype

7

Prototype: frequency response of the input stage

Network analyserPort1 Port2

50 Ω

-20dB

attenuator

Expected:

Measured:

150 MHz150 MHz100 KHz

1 GHz100 KHz

Resonances in the input stage.Out of our band of interest, so it will be

removed in the band limiter stage Highest frequency of ⱷ loop presently at 80 MHz (h=169): response in band of interest almost constant

This test shows the response of the -20dB version, other amplifiers have very similar response

8

Complete prototype with bandwidth limiter and output buffers

Network analyserPort1 Port2

20dB

attenuator

Band limiter

50 Ω

50 Ω50 Ω 1 GHz100 KHz

100 KHz 100 MHz

150 MHz

• Frequency response in band of interest quasi constant• Components at higher frequency attenuated

9

Tests with beam signal

Clipping under high intensity beam

Injection

Extraction

Transition

Low intensity beam1,48 s

Very high intensity beam

• High sensitive pickup• High input impedance amplifier

Bunch shape

10

LHCPROBE

TOF

TOF

Not saturated

Saturated and correctly clipped

Beam phase loop closed with the prototype amplifier

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ΔK Δⱷ

Injection

ExtractionTransition

Injection

ExtractionTransition

Acceleration a LHCPROBE with High sensitive pickup

Acceleration a LHCPROBE with prototypeΔK Δⱷ

50 mV/deg

• Phase loop closed on LHCPROBE beam

• Delay not compensated yet

This Δⱷ depends only by the

different delay

Δⱷ

ΔⱷW

CM

RF cavity

200 mV/div500 mV/div

Phase loop closed all along the cycle

Outlook and conclusion

• Assembly of complete setup of multiple amplifiers, in a 19" crate and installation in the PS RF Control Room

• Final tests with the amplifiers connected directly to the WCM and validation of prototypes foreseen in June

• Prototype successfully tested with beam signal (connected after the first passive splitter)• Beam phase loop closed using the prototype amplifier connected to the WCM95• Board layout being finalized in electronics design office (Thanks to Jean Marc Combe)

Present status:

Next steps:

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THANK YOU FOR YOUR ATTENTION!